The present invention relates to a multi-layer extrusion head and in particular to one for intermittent extrusion of multiple parisons.
While it has previously been known to intermittently extrude a single thermoplastic parison having multiple layers of resin, and it has also been known to simultaneously extrude multiple parisons of a single resin layer, the intermittent extrusion of multiple parisons having multiple resin layers from a single extrusion head has not previously been successfully performed. The apparatus of the present invention seeks to combine the intermittent multi-layer extrusion with a multiple parison extrusion head to achieve the added production capability of multiple parison extrusion with intermittently extruded multiple layer parisons. The challenge in doing so is to produce parisons which, to the greatest extent possible, are identical with one another regardless of from which station of the extrusion head the parison is extruded.
With an intermittent extrusion process, with the beginning of each extrusion cycle or shot, when the piston, plunger, or reciprocating screw begin to move to push the resin, the resin within the flow passages will undergo a certain amount of compression. As a result of the resin compression, there will be a delay between the time when the plunger etc. begins to move and the time when resin begins to flow from the outlet orifice of each extrusion station. This delay is referred to throughout this specification as the compression delay. The length of the compression delay is directly related to the volume of resin within the resin passages between the extruder screw or accumulator and the extrusion station within the extrusion head. The compression delay does not create a difficulty with a single parison multi-layer intermittent extrusion head since the start time for each piston etc. for each of the resin layers can be controlled, taking into account the compression delay so that the flow of resin for each layer from the outlet orifice of the extrusion head begins at the desired time to form the desired parison wall structure. However, with multiple parison extrusion, due to the added flow passages necessary to feed resin to multiple extrusion stations, the compression delay now presents a greater obstacle. If the compression delays are not properly accounted for in programming the machine sequence, defects can be formed along the entire length of the parison, not just at the beginning of each parison.
It is an object of the invention to provide an intermittent multi-layer multi-parison extrusion head which overcomes the difficulties presented by compression delay in the resin to produce nearly identical parisons from each of the extrusion stations.
In order to regulate the resin flow, the resin flow passages are typically equipped with flow plugs that can be adjusted providing varying degrees of restriction within the flow passages. In doing so, the plugs typically produce stagnation of the resin immediately behind the plug where the resin can dwell and ultimately degrade. It is a further object of the present invention to eliminate the pockets of stagnating material behind a flow plug while maintaining flow control. It is a still further object to provide flow control devices within the extrusion head which are accessible from the front of the extrusion head for ease in making adjustments.
One way to accommodate the compression delay and produce multiple, identical parisons is to provide the resin flow passages to each extrusion station with equal volumes of resin. With equal resin volume flow passages, the compression delay of each resin for each station will be identical. This can be accomplished in one of two ways. First, the longest flow passage can be made with the smallest diameter while the shortest passage is made with the largest diameter so that the volume of resin in each passages is equal. However, this has the disadvantage of dramatically increasing the pressure drop required due to the greater resistance to resin flow in the smaller diameter passages, resulting in slower extrusion rates.
Alternatively, equal resin volume can be produced by a continuous branching scheme where the main resin stream is divided into two and each of those streams are divided into two, etc., until the desired number of resin streams is reached. The resin volume between the extruder screw and each extrusion station will be equal. While such a branching scheme achieves the desired result of equal compression delay, the manifold necessary for supplying the resins to the extrusion stations is relatively large in size and is complex to manufacture. The continuous branching scheme can only be used with an extrusion head having 2, 4, 8, 16 etc. extrusion stations. It can not be used with an extrusion head having, for example, six stations or an odd number of stations. The continuous branching scheme, while achieving the first objective of the invention, is not a preferred embodiment.
Applicant has found that the volume of the flow passages, and hence the amount of the compression delay, need not be identical for all parisons but that the sequence of events for the extrusion of each parison must be the same if identical parisons are to be produced. For example, if in a three layer parison it is necessary for the two outer layers to begin flow slightly before the center layer to produce a satisfactory parison, this sequence must be followed in each of the multiple extrusion stations. While the compression delay depends upon the volume of resin in the flow passages, to have the same sequence of events for each station, it is sufficient that the volume ratios of the resin flow passages with respect to the layers within one parison be the same as for the other parisons. For example, with a three layer extrusion device, the ratio of the resin volume in the flow passages for the inner layer to the outer layer for one extrusion station must be the same, as for the other extrusion stations. While this will automatically be true for the continuous branching scheme in which the resin volumes from one station to the next are the same, a more simplified and compact passage configuration scheme can now be employed in which the resin volumes are not equal but the volume ratios are equal.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying claims.